until today, all my "online" index rebuilds worked as expected. but just today I got a series of transactions running and SQL is not able to get the LCK_M_SCH_M and end the rebuild.

so, for me, today it feels as "not exactly online index rebuild". I would put it at 10%.

If you had 1 failure out of 10 tries equating to 10% then by that accounting you can reasonably look forward to 90 error-free online rebuilds before your next failure...on average

Seriously though, the 99% was not meant to be thought of as a success-rate. If you have a very busy system, even though the rebuild only asks for short-lived stability-type locks at the beginning and end of the rebuild, i.e. the rebuild displays properties of an online process during 99% of its operations and only displays offline properties for 1% of its operations, you can still run into problems during that 1% on a very regular basis on a busy system.

__________________________________________________________________________________________________There are no special teachers of virtue, because virtue is taught by the whole community. --Plato